1. Field of the Invention
This invention relates to compositions useful for textile sizes, based on blends of particular poly(vinyl alcohol) copolymers with starches, and to the process of weaving textile fabrics which employs the compositions as sizes and utilizes their ready ability to be desized. The sizes are based on copolymers having a high level of acrylic ester comonomer, blended with various starches. Desizing involves use of caustic solutions and is ideally suited to such compositions.
2. Background of the Invention
Poly(vinyl alcohol) homopolymers, and certain poly(vinyl alcohol) copolymers have been known for use as textile sizes for many years. For convenience, both will be generically referred to hereinafter as PVA(s) or PVA polymers. When specificity requires they will be referred to as PVA homopolymers or homopolymer PVA and PVA copolymers or copolymer PVA. By convention, homopolymer PVA includes PVA derived from homopolymer poly(vinyl acetate) which has been only partially hydrolysed as well as that which has been `fully` (&gt;98%) hydrolysed. The terms `fully hydrolysed PVA homopolymer` and `partially hydrolysed PVA homopolymer` will be used when distinction is necessary. It is also possible to have fully or partially hydrolysed PVA copolymers though most copolymers are fully hydrolysed.
These different PVAs differ quite significantly in properties as textile sizes and in the ability of fabrics sized with them to be desized. This difference primarily depends on the degree of hydrolysis and the comonomer content, but also on other factors including molecular weight and thermal history.
PVAs are commonly prepared by alcoholysis of the corresponding poly(vinyl acetate) homopolymer or copolymer. The process is often (though not strictly correctly) referred to as hydrolysis; hence the term `partially hydrolyzed` when not all the acetate groups are completely converted to alcohol groups. When homopolymer poly(vinyl acetate) is only partially hydrolysed, the resulting PVA is really a vinyl alcohol/vinyl acetate copolymer. However, as noted, such polymers are generally referred to as PVA homopolymers. The term copolymer in this regard is reserved for materials which result from hydrolysis of the corresponding vinyl acetate copolymer, i.e. polymer also containing units derived from a monomer other than vinyl acetate.
Fully hydrolysed PVA homopolymer is highly crystalline, and strong, but because of its high crystallinity it dissolves only in hot, not cold water. Furthermore, when it is subjected to high temperatures, it can develop even higher levels of crystallinity than as prepared, resulting in polymer which is even more difficult to dissolve. Finishing mills with certain fabrics, particularly blend fabrics, tend to use a heat setting condition to relieve fiber stress. The treatment is typically carded out at temperatures which develop further crystallinity in fully hydrolysed PVA homopolymer, so that when such polymer is used as size on fabric, the treatment causes an increase in its crystallinity and a decrease in ease of subsequent desizing.
PVA copolymers and partially hydrolysed PVA homopolymers are less crystalline, and dissolve at lower temperatures, or more rapidly at a given temperature. As a result they desize in water more readily, and are less subject to change in crystallinity and ability to be desized with fabric heat-setting treatments. For a given level of comonomer or residual unhydrolyzed acetate units however, the two types of PVAs are not identical in several respects. This is partly because the distribution of comonomer units (or units derived from them by lactonization, as discussed below) along the polymer chain is not the same as the distribution of residual acetate units along the chain after partial hydrolysis.
Various PVA copolymers have been disclosed as being useful for textile sizes. U.S. Pat. No. 3,689,469 (Inskip et al.) discloses PVA copolymers with 2 to 6.5 weight percent methyl methacrylate as comonomer which are useful as textile sizes, and compares their properties as sizes with fully hydrolysed and partially hydrolysed PVA homopolymer. The disclosure also indicates however, that above about 6 weight percent methyl methacrylate such copolymers are excessively water soluble.
PVA copolymers containing 1 to 10 mole percent methyl acrylate or methyl methacrylate as comonomer are disclosed in U.S. Pat. No. 4,990,335 (Bateman et al.). (For methyl acrylate this corresponds to about 2 to 16 weight percent methyl acrylate in the polymer, calculated as non-lactonized vinyl alcohol copolymer). The polymers are disclosed as being useful for certain tableting applications. There is no suggestion for use of such polymers as a textile size.
Japanese Patent No. 75-32355 discloses modified poly(vinyl alcohol) polymer fiber sizing agents containing 0.1 to 15 mole % lactone rings. In an example, cotton fabric sized with a 4.7 mole % lactone polymer prepared by saponifying (hydrolyzing) a poly(vinyl acetate/methyl acrylate) copolymer with 4.5% methyl acrylate (which corresponds to 4.7 mole % lactone when the comonomer is fully lactonized, and to about 8.1 weight percent methyl acrylate calculated as non-lactonized vinyl alcohol copolymer) had better scouring fastness than homopolymer PVA.
U.S. Pat. No. 4,172,930 (Kajitani et al.) discloses a PVA copolymer as textile size where the comonomer is 0.1-10 mole percent of a diacid such as maleic and fumaric acids, but having no monoester, diester or anhydride of the diacid. Copolymers containing free acid can be extremely water sensitive.
Many other materials are known for use as textile sizes. Unmodified starches are inexpensive, but they do not generally have as good properties as PVAs, often flaking off the yarn when used as sizes. They do not give stable solutions, and often desizing requires use of enzymes. Many modified starches are known which are improvements in various ways over simple starches, but may be considerably more expensive. Polyacrylic sizes are also known and have good properties, but are extremely water sensitive. Known PVA based sizes may be considered to have, very generally, sizing properties intermediate between starches and polyacrylic sizes.
Blending different sizing materials is known and used. Blending can provide properties of the size itself, and economics, intermediate between those of the components. Heretofore however, PVA copolymers containing above 6.5 weight percent ester comonomer have not been considered as components of blend sizes. Nor has blending as a means of enhancing ability to be desized heretofore been considered.
Desizing usually involves water washing. However desizing of particular polymers using caustic solution is sometimes used and has been described. U.S. Pat. No. 4,013,805 (Corey et al.) discloses a poly(vinyl acetate) copolymer which contains a comonomer with free carboxylic acid groups derived from a monomer such as acrylic acid, which can be desized with aqueous base. The poly(vinyl acetate) copolymer is not hydrolysed to the corresponding poly(vinyl alcohol) copolymer.
Desizing of wax-free PVA polymer or copolymer sizes, where the copolymer may contain up to 6 weight percent methyl methacrylate or other comonomers, and wherein the size contains an alcohol ethoxylate surfactant, using an alkaline scour bath followed by hot water rinses, is disclosed as being easy compared with comparable sizes with wax but no surfactant, in U.S. Pat. No. 4,640,946 (Vassallo et al.).
Solubility and dissolution times of various types of PVA in water and caustic solutions are discussed in `Polyvinyl Alcohol`, John Wiley & Sons Ltd., 1992, Chapter 11, p365-368. It is noted there that partially hydrolysed PVA homopolymer dissolves more slowly in caustic solutions than in water, whereas PVA copolymers with methyl methacrylate as comonomer dissolve more rapidly in caustic than in water. This is explained by the fact that caustic further hydrolyses partially hydrolysed PVA to homopolymer, whereas with the copolymer, lactone tings known to be present are saponified, resulting in ionic groups which are highly soluble. The methyl methacrylate copolymers discussed were designated T-25 and T-66. The amounts of methyl methacryate in those copolymers were not disclosed. Those polymers are manufactured by E. I. du Pont de Nemours. They both contain less than 6.5 weight percent methyl methacrylate, calculated on the basis of non-lactonized poly(vinyl alcohol) copolymer.
The whole chapter in the above reference provides a general background to use of PVA copolymers and other materials in sizing applications.
Ease of desizing can strongly affect the economics of the weaving process. Many starches and modified starches are known for use as sizes. Each has a particular niche. Many are not readily de sized, do not have particularly good properties as sizes, and do not provide very stable solutions. There remains a need for sizes which have acceptable mechanical properties, give relatively stable size solutions, yet which have acceptable water insensitivity. A size material which can be used to upgrade starch size properties, but particularly to upgrade the ability to be desized, by utilization of such a material in blends with starch remains a strong need.